This invention relates to immersion thermocouple sensors used for multiple temperature measurements in molten irons, steels, and other metal baths during the process of manufacture, refining, or casting of such metals. More particularly, this invention relates to thermocouples which are intended for multiple use and which are cooled or kept at constant temperature internally by the flow of a gaseous cooling medium such as air.
Various immersion thermocouples are commercially available for use in measuring the temperatures of molten metal baths. Some thermocouples are intended for single use and others are intended for multiple use. One such multiple use thermocouple device is described in U.S. Pat. No. 4,521,639 issued Jun. 4, 1985. A single use thermocouple assembly is illustrated by U.S. Pat. No. 4,881,824 issued Nov. 21, 1989. Whenever and wherever possible, it is desirable, especially for cost considerations, to use an expendable thermocouple device as many times as possible in the molten metal bath. Varying the time between immersions can substantially affect the life of the thermocouple and the cost per temperature reading. Short times between immersions and/or longer immersion times can result in the build-up of excessive conductive and radiant heat with resultant overheating of the expendable thermocouple sensor cold junction and connector, permanent connector housing, and the immersion lance pole.
This overheating creates several very undesirable results, including but not limited to reducing the life and accuracy of the thermocouple, thus leading to unreliable temperatures even though temperatures continue to be obtained by the operator. A secondary thermocouple forms at the cold junction points between the thermocouple connector tip lead wires and the permanent connector ring and stem. Significant temperature changes at the cold junction and the resultant development of a secondary thermocouple can cause temperature readings to be inaccurate. The cold junction must remain at a relatively constant temperature in order to prevent secondary thermocouple affects from impairing the accuracy of the temperature measuring system. The continuity, life and accuracy of the system can also be destroyed by the melting of the insulating composition or other parts in the sensor, sensor connector, permanent connector and housing, lead wires, or other parts in the thermocouple pole assembly.
The components of the tip of the expendable device may contain paper, plastic, or a combination thereof which, if overheated, can melt or burn, thus destroying the device and its integrity prematurely. Also, the insulating housing of the permanent connector may melt or burn at high temperatures.